The problem of sound distortion in loudspeakers is well known. Generally it is detectable especially in the bass regions of sound reproduction as a form of "rumble", which muffles or masks the full purity of the bass tones. The problem also occur in the mid-range and upper ranges of audio frequency reproduction, but is less noticeable to an untrained ear. This distortion is apparent in coil-driven loudspeaker systems having a single coil-driven loudspeaker, as well as those having a plurality of coil-driven loudspeakers. High fidelity audio loudspeaker systems usually comprise at least two and more often, three or more separate coil-driven speakers. These speakers will include a speaker to cover the high frequency high notes (tweeter) and a speaker to cover the low frequency bass notes (woofer), and in most cases, a speaker to cover the mid-range frequency notes (mid-range). In some cases there may be multiple speakers for each range. It is customary in such multi-speaker systems to provide one or more filter circuits known as "crossovers" in which the signals for the various ranges are separated so that they are reproduced in the appropriate speakers in the system. Such crossovers incorporate one or more crossover coils as part of the filter circuit. The precise causes of the type of distortion described above are not entirely clear. However, it seems reasonable to assume that one source is the collapsing of the magnetic fields created around the crossover coil during the passage of audio signals. As the magnetic fields collapse, they induce, within the coil, a secondary transient signal related to, but not part of, the primary audio signal. Some evidence is available for this theory in the well-known relationship between the strength of the primary signal and the strength of the distortion signal. Various attempts have been made to deal with the problem.
One recent proposal is shown in U.S. Pat. No. 4,160,133. In this patent, the speaker itself is manufactured with an additional damping coil mounted directly on the speaker. The degree of effectiveness of this solution has not been evaluated, but it is certain that the cost of manufacturing speakers incorporating this proposal would be considerably higher than the manufacture of conventional speakers, and the efficiency of the speaker is adversely affected. Thus such a solution would be less than optimal for the consumer. Consequently, this proposal has not achieved wide acceptance.
In general terms, the present invention finds its application both to single speakers and to such crossover circuits for multiple speakers so that a damping effect is provided over a part of the frequency ranges or indeed all of the frequency ranges to damp out distortion.
It is believed that a major cause of speaker distortion is in the design of the crossover circuits themselves. Such crossover circuits inherently incorporate some form of coils, of varying inductances, whereby signals may be divided up into groups or bands of selected wavelengths for reproduction in the different speakers. It is, of course, well known that the passing of electrical current wave forms through a coil will result in the development of transient electromagnetic fields around the coil itself. As the current fluctuates, so also does the induced electromagnetic field. The fluctuation of the induced electromagnetic field is believed to induce, in turn, a fluctuating voltage across the coil which is passed through the speaker coil producing a further unwanted movement and hence sound waves from the speaker. It is believed that this is a major cause of the distortions or so-called "rumble" which can be heard in speaker systems and this distortion is generally considered to be undesirable by the great majority of listeners. While this explanation has not been scientifically proved it does seem to be a reasonable explanation for the phenomenon of sound distortion in speakers.
It will of course be understood that in most of the speaker systems to which the invention relates, the speakers will be of the moving coil type. Such speakers inherently incorporate their own integral coil means. Such speaker coils will in themselves develop a back EMF, induced as the voice coil moves through the magnetic field of the permanent magnet which surrounds the voice coil. This factor is a "given" in almost all speaker systems, and may also be, in itself, a cause of distortion.